The Pure Tone Average, commonly referred to as PTA, is a single number used by hearing health professionals to summarize an individual’s overall hearing ability in each ear. Its purpose is to provide a quick reference point for characterizing the severity of hearing loss. The PTA helps classify hearing loss into categories like mild, moderate, or severe, which aids in determining appropriate management.
Understanding Pure Tone Thresholds
The input data for calculating the Pure Tone Average comes from an audiogram, which is the graphical record of a hearing test. A pure tone threshold is defined as the softest sound intensity an individual can hear at a specific frequency at least 50% of the time.
These thresholds are measured in decibels hearing level (dB HL), a unit that represents the intensity of the sound relative to the average hearing of young, healthy adults. The process involves presenting a series of single-frequency sounds, or “pure tones,” through headphones or speakers. The audiogram plots the intensity (in dB HL) required for a person to hear each tone across a range of frequencies.
Step-by-Step Guide to the Standard Calculation
The standard method for determining the Pure Tone Average involves calculating the arithmetic mean of three specific hearing thresholds. This approach focuses on the frequencies most important for understanding human speech. The three frequencies used in this calculation are 500 Hz, 1000 Hz, and 2000 Hz.
The first step is to identify the measured hearing threshold, in dB HL, for each of these three speech frequencies in the ear being tested. Next, these three decibel values are summed. Finally, the sum is divided by three to yield the Pure Tone Average for that ear.
For example, if a patient’s thresholds are 30 dB HL at 500 Hz, 40 dB HL at 1000 Hz, and 50 dB HL at 2000 Hz, the calculation is \((30 + 40 + 50) / 3\). This results in a Pure Tone Average of 40 dB HL.
Alternative Averaging Methods and Clinical Significance
While the 3-frequency average is the standard, other methods are sometimes used to provide a more representative picture of hearing loss in specific contexts. One common alternative is the 4-frequency average, which includes the thresholds at 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz. This 4-frequency PTA is often employed in occupational health settings and for specific regulatory purposes, as the 4000 Hz frequency is highly susceptible to noise damage.
Another variation is the Fletcher Average, which accounts for steeply sloping hearing loss configurations by only averaging the best two of the three primary speech frequencies (500 Hz, 1000 Hz, and 2000 Hz). This method can provide a better correlation with a person’s ability to hear faint speech, especially when a high-frequency hearing loss is present.
Degree of Hearing Loss Classification
The calculated PTA is used to formally classify the degree of hearing loss. Common classifications include:
- Normal hearing: 0 to 25 dB HL
- Mild hearing loss: 26 to 40 dB HL
- Moderate hearing loss: 41 to 55 dB HL
- Severe hearing loss: 71 to 90 dB HL
The PTA also serves as a check for the accuracy of the Speech Recognition Threshold (SRT). The SRT is the softest level at which a person can correctly repeat speech, and typically the PTA and the SRT should align within about 5 to 10 dB.